1. Field of the Invention
This invention concerns a method of manufacturing an X-ray exposure mask (referred to hereafter as an X-ray mask), a device for controlling the internal stress of thin films, and in particular, the improvement of X-ray absorption thin film patterns.
2. Description of the Related Art
In recent years, there has been a strong demand for higher density of semiconductor integrated circuits and high integration, and with regard to precision processing of circuit patterns, rapid growth is taking place in the research and development of lithographic techniques to form patterns on photosensitive materials.
At present, photolithography which uses light as the exposure medium is the most widely employed but a limit to the resolving power that is possible with this technique is now being reached. In place of photolithography, therefore, intensive research is being carried out on X-ray lithography which in principle can offer drastic improvements in resolution.
In X-ray lithography, unlike the exposure method used with light, no technique has yet been devised to reduce any specified pattern in size before it is transferred. In X-ray lithography, therefore, an X-ray exposure mask with the specified pattern and the sample are held parallel at an interval of about 10 .mu.m apart, and the pattern is transferred to the surface to be exposed by irradiating it with X-rays through this mask with a 1:1 correspondence.
In this 1:1 transfer arrangement, the dimensional and positional precision of the pattern on the X-ray mask becomes the precision of the device, hence it is required that the dimensional and positional precision of the pattern on the mask is of the order of 1/10 of the minimum line width of the device. Further, as SOR (synchrotron orbital radiation) light is usually preferred as the X-ray source, the mask structure must be capable of withstanding strong X-ray radiation without sustaining damage. Moreover, with the narrowing of line widths from 0.5 .mu.m to the 0.1 .mu.m of the next generation of devices, the vertical/horizontal ratio in a section through a pattern on the mask is increasing, and this may be expected to lead to various structural problems.
The major requisite for the improvement of X-ray lithography at this time is thus the development of better structures for X-ray masks and of methods of manufacturing them.
X-ray masks usually have the following structure. A thin film of X-ray transmitting material, which has a very low absorption coefficient for X-rays, is formed on a ring-shaped mask support, and a mask pattern (X-ray absorber pattern) is formed from a material with a high X-ray absorptivity coefficient on this X-ray transmitting film. In this structure, the mask support is used to provide reinforcement for the X-ray transmitting film which is mechanically very weak owing to its very small thickness.
This X-ray exposure mask has been manufactured by the following method.
Firstly, a SiC film of thickness 2.7 .mu.m is formed on the front surface of a Si substrate by LPCVD at a substrate temperature of 1200.degree. C. Under these conditions, the SiC film has a polycrystalline structure, and its internal stress is 3.times.10.sup.8 dyn/cm.sup.2. Next, another SiC film is formed in the same way on the reverse surface of the Si substrate. In this case, the SiC film formed on the reverse surface of the substrate is used as the X-ray transmitting film. X-ray transmitting films, in general, are required not only to pass X-rays, but also to have excellent transmittance with respect to alignment light (visible and infrared light), and in addition, to have tensile stress so that they are self-supporting. Suitable materials so far reported to have these properties include BN, Si, and Ti, in addition to SiC.
Next, the central part of the SiC film on the reverse surface of the substrate is selectively removed, and a W film is formed on the SiC film on the front surface to act as an X-ray absorber. The properties required of this X-ray absorber are that it should have a high X-ray absorptivity coefficient at the exposure wavelength and a low internal stress, and that it should be easy to precisely process. Suitable materials so far reported to have these properties include Au, Ta, and WNx, in addition to W. It is moreover essential that the internal stress of the X-ray absorber is as low as about 1.times.10.sup.8 dyn/cm.sup.2, and it is formed by sputtering which permits the stress to be controlled.
Next, an SiNx film of which the internal stress has been controlled, is formed on the W film by sputtering. An electron beam resist is then coated on the SiNx film, and patterning is carried out by an electron beam so as to form the desired pattern on the resist.
Next, the SiNx film is selectively etched by dry etching using the patterned resist as a mask, and the W film is then selectively etched using the patterned resist and etched SiNx film as a mask.
Finally, the Si substrate is etched by wet etching with KOH or the like using the SiC film on the reverse surface as a mask. In this way, the X-ray mask can be produced.
In this manufacturing process, however, the most troublesome step is the formation of the X-ray absorber pattern.
X-ray absorption films capable of transferring dimensions of 0.5 .mu.m or less described above with high precision, are required to have the following properties:
(1) They must have high density. PA0 (2) They must be stable and without stress variation under the thermal step conducted in the fine patterning process. PA0 (3) They must have low stress (no more than 1.times.10.sup.8 dyn/cm.sup.2). PA0 (4) They must be capable of high precision processing.
All the above four criteria must be satisfied.
The following reports have been made concerning attempts to manufacture X-ray absorber patterns which satisfy these conditions.
W-Ti films can be formed in a DC magnetron sputtering device using W-Ti (1%) alloy as a sputter target in Ar+30%N.sub.2 gas atmosphere. At low gas pressures, these films show a compressive stress, but thin films sputtered at gas pressures of 2 Pa or more exhibit a small value of 5.times.10.sup.8 dyn/cm.sup.2. The density of the thin film is however 14-16 g/cm.sup.3, which is 20-30% less than that of pure tungsten (19.2 g/cm.sup.3) (Yoshioka et al, SPIE Conference, Vol. 923, p.2 (1988)).
Thus, to improve the precision of the pattern, a method has been proposed whereby the stress in the X-ray absorbing material is controlled to be less than 1.times.10.sup.8 dyn/cm.sup.2 by varying the sputtering conditions.
Also, it has also been demonstrated that the stress can be controlled by carrying out ion implantation over the whole surface of the X-ray absorber thin film. It is reported that if for example Si ions are implanted in a W film obtained by sputtering a W target in argon (Ar) gas, the stress can be reduced to nearly zero (I. Plotnik et al, Microelectronic Engineering 5, p. 51 (1986)). In this method, however, Si atoms or W-Si compounds at 1.times.10.sup.16 atoms/cm.sup.2 are generated as particles during etching, and tend to cause re-sticking or soiling by dirt, etc. Further, there is no description of how the W film implanted with Si ions behaves in precision processing, so there is still much uncertainty regarding the feasibility of obtaining high accuracy.
There is also a problem concerning the thermal stability of stress with the W films containing Si. In the processing of W films, resist baking is carried out at a temperature of 150-200.degree. C. At this temperature, there is a possibility that uncombined Si atoms which have been implanted in the W may form linkages and it is thought to be unlikely that the W film can remain stable with low stress during the heating process.
In another report, tungsten nitride (WNx) is used as absorber material, this being formed by sputtering in a mixture of argon gas and nitrogen gas, and N.sup.+ ions are implanted in the WNx to control stress (Journal of Vacuum Science Technology, B6(1), 174, 1988). Various attempts were made to reduce the stress of the X-ray absorber below 1.times.10.sup.8 dyn/cm.sup.2. However, the density of the WNx film is very low. It is therefore necessary to make the film thick if it is to provide an effective screen for X-rays as an X-ray absorber, and consequently, it is difficult to form a fine pattern therein. Moreover, to control the stress not more than 1.times.10.sup.8 dyn/cm.sup.2, the substrate temperature must be controlled to 300.degree. C..+-.7.degree. C. during ion implantation.
Further, even if a thin film pattern could be formed with stress control, the fact that metals such as tungsten or metal alloys were used for the X-ray absorber meant that on exposure to the atmosphere, oxidation or gas adsorption occurred and inevitably caused stress variations.
There is, for example, the following report concerning the stability of thin films formed by sputtering of a W-Ti target in Ar+30%N2 gas. After storage in the atmosphere for 2 months, the stress shifted in the direction of compressive stress by 1.times.10.sup.9 dyn/cm.sup.2, and after annealing in a N.sub.2 atmosphere at 200.degree. C. for 1 hour, there was a large shift of as much as 2.times.10.sup.9 dyn/cm.sup.2 in the direction of tensile stress.
In other words, there was a time-dependent variation of the stress in the X-ray absorber material, and the precision of the pattern of the absorber could not be maintained.
To protect the X-ray absorber pattern from oxidation or gas adsorption and to give some mechanical protection, a structure has been proposed wherein the X-ray absorber pattern is covered by a protecting film (International Electronics Meeting; 42, 1982).
In this method, however, not only the stress in the X-ray absorber film but also the stress in the protecting film had to be controlled, and it was therefore not suitable in practice.
In conventional X-ray exposure masks, therefore, it was extremely difficult to obtain an X-ray absorber film which fully satisfied the functions or properties of an X-ray absorber material (high density, low stress, stress stability and ability to be precision machined).